Method for plant and separating air by cryogenic distillation

ABSTRACT

In a plant for separating air which does not comprise an argon column, an intermediate-pressure column ( 102 ) has a bottom reboiler ( 24 ) which is heated by a gas ( 233 ) coming from the low-pressure column ( 103 ). The intermediate-pressure column is fed from the high-pressure column ( 101 ). This makes it possible to reduce the energy consumption while improving the efficiency of the process.

[0001] The present invention relates to a process and a plant forseparating air by cryogenic distillation. In particular it relates to aprocess using three separation columns operating at a high pressure, alow pressure and a pressure which is intermediate between the high andlow pressures.

[0002] It is known from EP-A-0538118 to use a process of this type inorder to separate air, the intermediate-pressure column having a bottomreboiler heated by nitrogen from the high-pressure column, thus reducingthe heating of the bottom reboiler from the low-pressure column.

[0003] One aim of the invention is to reduce the energy consumption ofthe separation process with respect to the processes of the prior art.

[0004] Another aim of the invention is to produce oxygen with a purityof at least 95 mol %, or even at least 98 mol %, with an improved yield.

[0005]FIG. 1 shows a conventional process with a low-pressure column 103operating at 1.3 bara enabling oxygen to be produced at 99.5 mol % witha yield of 92%.

[0006] A stream of 1 000 Nm³/h of air 1 at about 5 bara is divided intotwo in order to form a first stream 17 and a second stream 3 which issupercharged in a super-charger 5 at a higher pressure of about 75 bara.

[0007] The two streams 3, 17 are cooled on passing through a heatexchanger 100. The stream 17 is sent to the bottom of the high-pressurecolumn 101 and the liquefied stream 3 in the heat exchanger 100 isexpanded in a turbine 6 producing an at least partially liquid stream atits outlet, the fluid or mixture of fluids leaving the turbine 6 beingsent at least in part to the high-pressure column 101.

[0008] A rich liquid stream 10 from the high-pressure column 101 iscooled in the subcooler 83 before being expanded and sent to anintermediate level of the low-pressure column 103.

[0009] A liquid airstream 12 is withdrawn from the high-pressure column101, cooled in the subcooler 83, expanded and sent to the low-pressurecolumn 103.

[0010] A waste nitrogen stream 72 is withdrawn from the top of thelow-pressure column 103, sent to the subcooler 83 and then to the heatexchanger 100 where it is warmed.

[0011] A stream 31 of 193 Nm³/h of oxygen at 99.5 mol % is withdrawn inliquid form from the low-pressure column 103, pumped in the pump 19 to40 bara and vaporized in the heat exchanger 100 in order to form apressurized gas stream.

[0012] A stream of 200 Nm³/h of gaseous nitrogen 33 is withdrawn fromthe top of the high-pressure column 101 and is partially heated in theheat exchanger 100. At an intermediate temperature, part of the gas isexpanded in a turbine 35 before being mixed with the waste gas 72.

[0013] In another conventional diagram illustrated in FIG. 2, thelow-pressure column operates at 4.8 bara and the high-pressure column101 operates at 14.3 bara. This process produces oxygen at 99.5 mol %with a yield of 78%.

[0014] A flow of 1 000 Nm³/h of air 1 at about 14.3 bara is divided intotwo in order to form a first stream 17 and a second stream 3 which issupercharged in a super-charger 5 to a higher pressure of about 75 bara.

[0015] The two streams 3, 17 are cooled on passing through a heatexchanger 100. The stream 17 is sent to the bottom of the high-pressurecolumn 101 and the liquid stream 3 is expanded in a turbine 6 producingan at least partially liquid stream at its outlet, the fluid or mixtureof fluids leaving the turbine 6 being sent at least in part to thehigh-pressure column 101.

[0016] A rich liquid stream 10 from the high-pressure column 101 iscooled in the subcooler 83 before being expanded and sent to anintermediate level of the low-pressure column 103.

[0017] A liquid airstream 12 is withdrawn from the high-pressure column101, cooled in the subcooler 83, expanded and sent to the low-pressurecolumn 103.

[0018] A waste nitrogen stream 72 is withdrawn from the top of thelow-pressure column 103, sent to the subcooler 83 and then to the heatexchanger 100 where it is warmed.

[0019] A stream 31 of 164 Nm³/h of oxygen at 99.5 mol % is withdrawn inliquid form from the low-pressure column, pumped in the pump 19 to 40bara and vaporized in the heat exchanger 100 in order to form apressurized gas stream.

[0020] No gaseous nitrogen stream is withdrawn from the top of thehigh-pressure column 101 (of course a high-pressure gaseous nitrogenstream is condensed conventionally in a reboiler-condenser associatedwith the low-pressure column).

[0021] It is known from EP-A-833118 and U.S. Pat. No. 5,657,644 to heatan intermediate-pressure column of a triple-column system with anargon-enriched gas which also serves to feed an argon-production column.

[0022] The inventors of the present application have discovered that,even without using an argon-separation column, purification of theoxygen at the bottom of the low-pressure column remains satisfactory forthe production of high-purity oxygen.

[0023] According to one object of the invention, provision is made for aprocess for separating air in a separation apparatus comprising ahigh-pressure column, an intermediate-pressure column having a bottomreboiler and a low-pressure column in which

[0024] a) at least one mixture of at least oxygen, nitrogen and argon issent at least to the high-pressure column where it is separated into afirst oxygen-enriched stream and a first nitrogen-enriched stream,

[0025] b) at least part of the first oxygen-enriched stream is sent tothe column operating at intermediate pressure where it is separated intoa second oxygen-enriched stream and a second nitrogen-enriched stream,

[0026] c) at least part of the second oxygen-enriched stream and/or thesecond nitrogen-enriched stream is sent to the low-pressure column,

[0027] d) a gas is sent from the lower part of the low-pressure columnto the bottom reboiler of the intermediate-pressure column where it iscondensed at least partially before being sent back to the low-pressurecolumn,

[0028] e) at least one oxygen-enriched fluid and at least onenitrogen-enriched fluid are withdrawn from the low-pressure column and

[0029] f) at least part of the first nitrogen-enriched fluid iscondensed at least partially in a reboiler-condenser associated with thelow-pressure column and at least part of the at least partiallycondensed fluid is sent back to the high-pressure column

[0030] characterized in that no fluid containing between 3 and 20 mol %argon is enriched with argon in a column of the apparatus other than thehigh-pressure, low-pressure and intermediate-pressure columns.

[0031] According to other optional objects of the invention, provisionis made so that:

[0032] the oxygen-enriched fluid withdrawn from the low-pressure columncontains at least 95 mol % oxygen, possibly at least 98 mol % oxygen.

[0033] no nitrogen-enriched gas stream is withdrawn from the top of thehigh-pressure column or a nitrogen-enriched gas stream is withdrawn fromthe top of the high-pressure column.

[0034] the low-pressure column operates at at least 1.3 bara, optionallyat least 2 bara, preferably at least 4 bara.

[0035] one or more of the gaseous and/or liquid airstream(s) is (are)sent to the intermediate-pressure column and/or to the low-pressurecolumn and/or to the high-pressure column.

[0036] the gas coming from the lower part of the low-pressure columnsent to the bottom reboiler contains between 1 and 20 mol % argon,preferably between 5 and 15 mol % argon, even more preferably between 8and 10 mol % argon.

[0037] at least part of the second nitrogen-enriched stream iscondensed, optionally in a top condenser of the intermediate-pressurecolumn.

[0038] According to another object of the invention, provision is madefor a plant for separating air by cryogenic distillation comprising ahigh-pressure column, an intermediate-pressure column having a bottomreboiler and a low-pressure column, the high-pressure column and thelow-pressure column being thermally coupled together, means for sendinga mixture of at least oxygen, nitrogen and argon at least to thehigh-pressure column, means to send an oxygen-enriched stream from thehigh-pressure column to the intermediate-pressure column, means to sendan oxygen-enriched fluid and/or a nitrogen-enriched fluid from theintermediate-pressure column to the low-pressure column, means to send afluid from the low-pressure column to the bottom reboiler of theintermediate-pressure column, means to withdraw a nitrogen-enrichedfluid and an oxygen-enriched fluid from the low-pressure columncharacterized in that it does not comprise means for the argonenrichment of a fluid containing between 3 and 20 mol % argon other thanthe high-pressure, low-pressure and intermediate-pressure columns.

[0039] According to other optional objects of the invention, the plantcomprises:

[0040] an expansion turbine and means to direct a stream from thelow-pressure column to this turbine without compressing the stream.

[0041] means to direct an airstream to the intermediate-pressure and/orlow-pressure and/or high-pressure column.

[0042] Optionally, the fluid sent to the reboiler is withdrawn from thelow-pressure column at a level lower than the level at which anoxygen-enriched fluid coming from the intermediate-pressure column isintroduced.

[0043] Preferably, the intermediate-pressure column has a top condenser.

[0044] The so-called “oxygen-enriched” or “nitrogen-enriched” fluids areenriched with these components with respect to air.

[0045] Implementation examples of the invention will now be describedwith respect to FIGS. 3 and 4, which show schematic drawings of a plantaccording to the invention.

[0046] In FIG. 3, the apparatus operates with a low-pressure column at1.3 bara and in FIG. 4, the apparatus operates with a low-pressurecolumn at 4.8 bara.

[0047] The plant of FIG. 3 comprises a high-pressure column 101operating at 5 bara, an intermediate pressure column 102 operating at2.7 bara and a low-pressure column 103 operating at 1.3 bara. Part ofthe gaseous nitrogen from the top of the high-pressure column serves toheat the bottom reboiler of the low-pressure column but other heatingmeans can be envisaged, such as double reboiler systems, one of which isheated by air.

[0048] A stream of 1 000 Nm³/h of air 1 at about 5 bara is divided intotwo in order to form a first stream 17 and a second stream 3 which issupercharged in a super-charger 5 to a higher pressure of about 75 bara.

[0049] The two streams 3, 17 are cooled on passing through a heatexchanger 100. The stream 17 is sent to the bottom of the high-pressurecolumn 101 without having been expanded or compressed and the liquidstream 3 is expanded in a turbine 6 producing an at least partiallyliquid stream at its outlet, the fluid or mixture of fluids leaving theturbine 6 being sent at least in part to the high-pressure column 101.

[0050] A rich liquid stream 10 from the high-pressure column 101 iscooled in the subcooler 83 before being expanded and sent to anintermediate level of the intermediate-pressure column 102 between twosections, for example of structured packings of the crossed-corrugatedtype. The liquid can be sent to another level of the column and thecolumn can also receive a gaseous air or liquid stream.

[0051] This liquid is separated into a second oxygen-enriched liquid 20and a nitrogen-enriched liquid 25. The liquid 25 is cooled in thesubcooler 83, before being expanded and sent to the top of thelow-pressure column 103, after being mixed with a stream of lean liquid15 from the top of the high-pressure column 101 which has also beencooled in the subcooler 83 and expanded in a valve.

[0052] The liquid 20 from the bottom of the intermediate-pressure columnis divided into two. Part is expanded and sent directly to thelow-pressure column while the rest is expanded in a valve, sent to thetop condenser 22 of the intermediate-pressure column where it isvaporized at least partially before being sent to the low-pressurecolumn 103.

[0053] A liquid airstream 12 is withdrawn from the high-pressure column,cooled in the subcooler 83, expanded and sent to the low-pressure column103.

[0054] The reboiler 24 at the bottom of the intermediate-pressure column102 is heated by means of an argon-enriched gas stream 233 containingabout 5 to 15 mol %, preferably between 8 and 10 mol %, argon from thelow-pressure column 103. This stream is condensed at least partially inthe reboiler 24 before being sent back to the low-pressure column 103.

[0055] A waste nitrogen stream 72 is withdrawn from the top of thelow-pressure column 103, sent to the subcooler 83 and then to the heaterexchanger 100 where it is warmed.

[0056] A stream 31 of 203 Nm³/h oxygen at 99.5 mol % is withdrawn inliquid form from the low-pressure column 103, pumped in the pump 19 to40 bara and vaporized in the heat exchanger 100 in order to form apressurized gas stream.

[0057] A stream 33 of 200 Nm³/h of gaseous nitrogen is withdrawn at thetop of the high-pressure column 101 and is partially heated in the heatexchanger 100. At an intermediate temperature, part of the gas isexpanded in a turbine 35 before being mixed with the waste gas 72. Therest of the nitrogen continues its reheating and is a product of theapparatus.

[0058] It is possible to withdraw liquid products from the apparatus butthe apparatus does not produce any argon-rich fluid.

[0059] The plant of FIG. 4 comprises a high-pressure column 101operating at 14.3 bara, an intermediate-pressure column 102 operating at8.5 bara and a low-pressure column 103 operating at 4.8 bara. All thegaseous nitrogen from the top of the high-pressure column serves to heatthe bottom reboiler of the low-pressure column but other heating meanscan be envisaged, such as systems with double reboilers, one of which isheated by air.

[0060] A stream of 1 000 Nm³/h of air 1 at about 14.3 bara is dividedinto two in order to form a first stream 17 and a second stream 3 whichis supercharged in a super-charger 5 to a higher pressure of about 75bara.

[0061] The two streams 3, 17 are cooled on passing through a heatexchanger 100. The stream 17 is sent to the bottom of the high-pressurecolumn 101 and the liquid stream 3 is expanded in a turbine producing anat least partially liquid stream at its outlet, the fluid or mixture offluids leaving the turbine being sent at least in part to thehigh-pressure column 101.

[0062] A rich liquid stream 10 from the high-pressure column 101 iscooled in the subcooler 83 before being expanded and sent to anintermediate level of the intermediate-pressure column 102 between twosections, for example of structured packings of the cross-corrugatedtype. The liquid can be sent to another level of the column and thecolumn may also receive a stream of gaseous or liquid air.

[0063] This liquid is separated into a second oxygen-enriched liquid 20and a nitrogen-enriched liquid 25. The liquid 25 is cooled in thesubcooler 83, before being expanded and sent to the top of thelow-pressure column 103, after being mixed with a lean liquid stream 15from the top of the high-pressure column 101 which has also been cooledin the subcooler 83 and expanded in a valve.

[0064] The liquid 20 from the bottom of the intermediate-pressure columnis divided into two. Part is expanded and sent directly to thelow-pressure column while the rest is expanded in a valve, sent to thetop condenser 22 of the intermediate-pressure column where it isvaporized at least partially before being sent to the low-pressurecolumn 103.

[0065] A liquid air flow 12 is withdrawn from the high-pressure column,cooled in the subcooler 83, expanded and sent to the low-pressurecolumn.

[0066] The bottom reboiler 24 of the intermediate-pressure column 102 isheated by means of an argon-enriched gas stream 233 containing about 5to 15 mol %, preferably 8 to 10 mol %, argon coming from thelow-pressure column 103. This stream is condensed at least partially inthe reboiler 24 before being sent back to the low-pressure column 103.

[0067] A waste nitrogen stream 72 is withdrawn from the top of thelow-pressure column 103, sent to the subcooler 83 and then to the heatexchanger 100 where it is warmed.

[0068] A stream 31 of 177 Nm³/h oxygen at 99.5 mol % is withdrawn inliquid form from the low-pressure column, pumped in the pump 19 to 40bara and vaporized in the heat exchanger 100 in order to form apressurized gas stream.

[0069] It is possible to withdraw liquid products from the apparatus butthe apparatus does not produce any argon-enriched fluid.

[0070] The advantages of the invention will appear clearly on studyingthe table below.

[0071] Other alternative or additional refrigerating means can beenvisaged, such as an air-blowing turbine, a Claude turbine or anotherturbine which is not fed by a liquid stream or a gas turbine from thelow-pressure column.

[0072] The apparatus may receive all or part of its feed air from acompressor of a gas turbine, the waste nitrogen from the apparatus beingsent back to the gas turbine. Process of Process of (invention) Pressureof the high- 5 bara 5 bara pressure column Pressure of the low-pressure1.3 bara 1.3 bara column Pressure of the 2.7 bara intermediate-pressurecolumn Total airstream treated 1 000 Nm³/h 1 000 Nm³/h Oxygen content ofthe 99.5% O₂ 99.5% O₂ gaseous product Oxygen production considered 193Nm³/h 203 Nm³/h pure High-pressure gaseous 200 Nm³/h 200 Nm³/h nitrogenproduction Efficiency of extraction of 92% 97% oxygen Separation energyBase: 100 95

[0073] Process of Process of (invention) Pressure of the high- 14.3 bara14.3 bara pressure column Pressure of the low-pressure 4.8 bara 4.8 baracolumn Pressure of the 8.5 bara intermediate-pressure column Totalairstream 1 000 Nm³/h 1 000 Nm³/h Oxygen content of the 99.5% O₂ 99.5%O₂ gaseous product Oxygen production considered 164 Nm³/h 177 Nm³/h pureHigh-pressure gaseous 0 Nm³/h 0 Nm³/h nitrogen production Efficiency ofextraction of 78% 85% oxygen Separation energy Base: 100 90

1. Process for separating air in a separation apparatus comprising a high-pressure column (101), an intermediate-pressure column (102) having a bottom reboiler (24) and a low-pressure column (103) in which a) at least one mixture (1) of at least oxygen, nitrogen and argon is sent at least to the high-pressure column where it is separated into a first oxygen-enriched stream and a first nitrogen-enriched stream, b) at least part of the first oxygen-enriched stream (10) is sent to the column operating at intermediate pressure where it is separated into a second oxygen-enriched stream (20) and a second nitrogen-enriched stream (25), c) at least part of the second oxygen-enriched stream and/or the second nitrogen-enriched stream is sent to the low-pressure column, d) a gas (233) is sent from the lower part of the low-pressure column to the bottom reboiler of the intermediate-pressure column where it is condensed at least partially before being sent back to the low-pressure column, e) at least one oxygen-enriched fluid (31) and at least one nitrogen-enriched fluid (72) are withdrawn from the low-pressure column and f) at least part of the first nitrogen-enriched fluid is condensed at least partially in a reboiler-condenser associated with the low-pressure column and at least part of the at least partially condensed fluid is sent back to the high-pressure column characterized in that no fluid containing between 3 and 20 mol % argon is enriched with argon in a column of the apparatus other than the high-pressure, low-pressure and intermediate-pressure columns.
 2. Process according to claim 1 in which the oxygen-enriched fluid (31) withdrawn from the low-pressure column contains at least 95 mol % oxygen, possibly at least 98 mol % oxygen.
 3. Process according to claim 1 or 2 in which no nitrogen-enriched gas stream is withdrawn from the top of the high-pressure column (101).
 4. Process according to claim 1 or 2 in which a nitrogen-enriched gas stream (33) is withdrawn from the top of the high-pressure column (101).
 5. Process according to claim 1, 2, 3 or 4 in which the low-pressure column (103) operates at at least 1.3 bara, optionally at least 2 bara, preferably at least 4 bara.
 6. Process according to one of the preceding claims in which one or more of the gaseous and/or liquid airstream(s) is (are) sent to the intermediate-pressure column and/or to the low-pressure column and/or to the high-pressure column.
 7. Process according to one of the preceding claims in which the gas (233) coming from the lower part of the low-pressure column sent to the bottom reboiler contains between 1 and 20 mol % argon.
 8. Process according to one of the preceding claims in which at least part of the second nitrogen-enriched stream is condensed, optionally in a top condenser (22) of the intermediate-pressure column.
 9. Plant for separating air by cryogenic distillation comprising a high-pressure column (101), an intermediate-pressure column (102) having a bottom reboiler (24) and a low-pressure column (103), the high-pressure column and the low-pressure column being thermally connected together, means for sending a mixture (1) of at least oxygen, nitrogen and argon at least to the high-pressure column, means to send an oxygen-enriched stream (10) from the high-pressure column to the intermediate-pressure column, means to send an oxygen-enriched fluid (20) and/or a nitrogen-enriched fluid (25) from the intermediate-pressure column to the low-pressure column, means to send a fluid (233) from the low-pressure column to the bottom reboiler of the intermediate-pressure column, means to withdraw a nitrogen-enriched fluid (72) and an oxygen-enriched fluid (31) from the low-pressure column characterized in that it does not comprise means for the argon enrichment of a fluid containing between 3 and 20 mol % argon other than the high-pressure, low-pressure and intermediate-pressure columns.
 10. Plant according to claim 9 comprising an expansion turbine and means to direct a stream from the low-pressure column to this turbine without compressing the stream.
 11. Plant according to claim 9 or 10 comprising means to direct an airstream to the intermediate-pressure and/or low-pressure and/or high-pressure column (101, 102, 103).
 12. Plant according to one of claims 9 to 11 in which the fluid (233) sent to the reboiler is withdrawn from the low-pressure column at a level lower than the level at which an oxygen-enriched fluid coming from the intermediate-pressure column is introduced.
 13. Plant according to one of claims 9 to 12 in which the intermediate-pressure column (102) has a top condenser (22). 